Terminology and Introduction
As used herein, the term "blank" refers to a lens element at any stage in the manufacture of the lens up to and including its finishing to a specified optical prescription but prior to edging to fit a particular spectacle frame. A "major blank" is the basic element of a composite glass lens, the major portion of the surface of which defines the basic optical property of the lens such as the distance correction. A "minor blank" is a minor component of a composite glass lens which is combined with a major blank to produce a desired optical effect. The term "blank" also refers to a mold element for casting a resin lens at any stage in the manufacture of a mold useful to cast a surface of a semi-finished or finished resin lens.
A "segment" is a portion of a glass or resin lens (or of a resin lens mold) which has an optical curvature or other optical or optically related characteristic different from remaining or other portions of the lens (or mold), such as the portion of a bifocal lens which defines a reading prescription as opposed to a distance prescription. An "addition" is a localized portion of a glass or resin lens which has an optical curvature or other optical characteristic different from remaining or other portions of the lens; a "reading addition" usually refers to the close or reading prescription portion of a multifocal lens. The terms "segment" and "addition" are often used interchangeably in describing lenses.
A lens has "positive" or "plus" curvature when it is of convex overall curvature, and has "minus" or "negative" curvature when it is concavely curved overall.
A "diopter" is a measure of the light focusing properties of a lens; a lens has a power of 1 diopter when it focuses a ray of incident light from a source of infinite distance from the lens at a point 1 meter behind the lens; a 2 diopter lens focuses an incident infinite-source ray 1/2 meter behind the lens. "Prism diopter" is a measure of the prism or light deviating effect of a lens. One prism diopter corresponds to the deviation of an emergent ray by 1 centimeter from the lens axis at a distance of 1 meter from the lens; that portion of a 1 diopter lens which lies 10 millimeters from the lens' optical axis produces a prismatic effect of 1 prism diopter.
It will be understood that any optical lens is designed to produce a particular optical effect when mounted in a particular manner to act upon light incident on the lens in a particular way. Virtually every optical lens is designed with these usage conditions in mind, regardless of whether the lens is for a camera, a telescope or an automobile headlamp. This is especially the case with ophthalmic lenses which are designed to be worn in a specified position by a particular person suffering from a specified optical difficulty to produce for that person a desired optical effect. Therefore, especially in the case of ophthalmia to which this invention pertains, it is not very helpful to consider a lens in the abstract; it is more helpful and useful to consider the lens in the context of its intended use. Similarly, an appreciation of molds for casting resin ophthalmic lenses is aided by keeping in mind the lens produced from a given mold and the effect of that lens on a person when properly fitted to the person.
In the context of this invention, the "optical axis" of an ophthalmic lens is the line along which the user of the lens looks when the lens is properly fitted to the user and the user is looking straight ahead. The optical axis of a resin ophthalmic lens will usually have a counterpart in the mold from which the lens is cast.